Model and report changes

1. The model has the ability to incorporate estimates of community prevalence, by region and age group, from the Office of National Statistics COVID-19 Infection Survey (see Data Sources for details). These are included weekly since the outset of the Survey in May 2020 for the age groups >4 years to inform trends in incidence that are too recent to be captured by the data on deaths.
2. The model now accounts for the ongoing immunisation programme, stratifying the population of people still susceptible to infection with the virus according to their immunisation status (unimmunised/1 dose/2 doses). We use data on the daily proportions of the population getting immunised to inform this splitting of the population, assuming that it takes three weeks for vaccine-derived immunity to develop .
3. The geographical definition has been changed from the seven NHS regions (map) to the nine regions typically used in government (map). This new spatial definition more appropriately reflects the existing regional heterogeneity.
4. Using observations of improved survival in hospitalised COVID-19 patients, we have allowed the probability of dying following infection with SARS-CoV2 (the infection-fatality rate, IFR) to gradually change over the course of June 2020, with a decrease being estimated. More recently, the Kent variant of the virus has gradually become the predominant virus strain and we accordingly allow for a change in the IFR over the period in which the relative prevalence of this strain has been growing.
5. The ‘Epidemic summary’ now only reports the current value for the IFR by age. To visualise how this has changed over time in our model, see the IFR tab in the ‘Infections and Deaths’ section of the report. The quantity that is now plotted under this tab is the probability of dying if infected, taking into account the impact of the immunisation programme.
6. The modelling now accounts for a different susceptibility to infection in the under-15s, using information from literature (Viner et al, 2020) suggesting that children less likely to acquire infection when in contact with an infectious individual.

Updated findings

1. The current estimate of the daily number of new infections occurring each day across England is 3,840 (2,510–5,9430, 95% credible interval).
2. The daily infection rate is estimated to be the highest in London (GL) with 783, new daily infections, corresponding to 9 per 100,000 population per day. All other regions have 6–7 new infections per 100k population with the exception of the North West where incidence appears low. Note these regional estimates are highly uncertain and that a substantial proportion of these daily infections will be asymptomatic.
3. We predict that the number of deaths occurring daily is likely to remain low with a forecast for the period around the 28th May suggesting that there will be between 9 and 35 deaths per day.
4. The probability of Rt exceeding 1 is 70% and 67% in the South West (SW) and GL respectively; 59% in both the East of England (EE) and South East (SE); around 30% in the West and East Midlands (WM and EM), North East (NE) and Yorkshire and Humber (YH); and less than 5% in the NW.
5. The growth rate for England remains at 0.01 (-0.01–0.03, 95% credible interval) per day. This means that, nationally, the number of infections is likely to be increasing, although there is considerable uncertainty and heterogeneity across regions, with negative growth in many, the NW in particular.
6. London, followed by the WM and the NE, have the highest attack rates, that is the proportions of the regional populations who have ever been infected, with 32%, 21% and 20% respectively. The SW continues to have the lowest attack rate at 10%. These constitute a big downward revision from our previous published report, particularly so for the NW.
7. Note that the deaths data used are only very weakly informative on Rt over the last two weeks and are thankfully becoming increasingly sparse. Therefore, the estimate for current incidence, Rt and the forecast of daily numbers of deaths are likely to be subject to some revision.

Interpretation

The plots of the estimated Rt in the most recent weeks are heavily influenced by the effects of the reopening of schools following the Easter holidays and the gradual relaxation of pandemic mitigation measures. Going forward, however, as restrictions continue to be relaxed, we anticipate Rt to remain stable. The Rt for four regions have central estimates just above 1 (EE, GL, SE, SW), although these estimates are uncertain. At current levels of incidence, these values of Rt are not a particular concern, though they do require careful monitoring.

The incidence of deaths has continued to fall more sharply than predicted by the model and is anticipated to continue to fall for the coming three weeks, despite the number of new infections, as the Rt values indicate, remaining flat in almost all regions.

The plot of the infection fatality rate (IFR) presents age-specific probabilities of death given infection. It shows an increasing mortality risk from September onwards in all ages until the immunisation programme begins to have an impact in late January. From the end of January we estimate a decreasing IFR in all adult age groups, but most steeply in the older ages. This drop measures the benefits of immunisation against death over and above the benefits against infection. Specifically, there is an estimated fall to a still-high 15% in the over-75s and 0.35% overall. The overall impact of the immunisation programme can be seen more clearly in the ‘All Ages’ plot, where the precipitous decline in IFR since late January is a product of this efficacy against death but also of the increasing proportion of infections in young people as older age groups are immunised and become protected against infection. The impact of the second immunisation doses becoming widespread will begin to affect this quantity over the coming weeks.

Estimates of cumulative infection are low in comparison to some earlier reports. This is due to the inclusion of the prevalence data, which appear to have the effect of reducing the number of infections. Nowhere is this more true than in the North West, where estimates of attack rate have fallen to 17%. London remains the region with the largest levels of cumulative infection to date.

Other indicators (e.g. hospital bed prevalence, reported new cases) continue to suggest a declining epidemic. Prevalence of infection, as estimated by the ONS Community Infections Survey is now around 0.10% in England with some regional heterogeneity. It is hoped that these trends continue, enabling the continued progressive relaxation of pandemic mitigation measures in line with the government’s roadmap to opening society. We will continue to monitor the situation closely.